The current study explored the associations between NHHR and both NAFLD and hepatic fibrosis, with findings indicating a positive relationship between NHHR and NAFLD across all models. However, while NHHR was positively correlated with liver fibrosis in all models, this correlation was not statistically significant in Model 3. Notably, an S-shaped association was observed between NHHR and NAFLD, with a critical inflection point identified at 2.49.
The rising prevalence of NAFLD underscores the urgent need for accurate, non-invasive diagnostic methods, particularly as liver biopsy, while the gold standard for diagnosis and staging, is invasive, costly, and carries a risk of complications [29]. VCTE has gained prominence as a leading non-invasive alternative, widely recognized for its reliable assessment of hepatic steatosis and fibrosis through the measurement of CAP and LSM [30, 31]. These techniques have demonstrated a high degree of accuracy, comparable to that of liver biopsy, making them essential tools in the clinical management of NAFLD, especially in scenarios requiring repeated monitoring [32].
The pathogenesis of NAFLD is multifactorial, with IR and dysregulated lipid metabolism being central to disease progression [4, 33]. Previous studies have established that metabolic syndrome, obesity, and type 2 diabetes mellitus (T2DM) are significant risk factors for NAFLD [34–36]. In the liver, IR promotes de novo lipogenesis, impairs triglyceride clearance, and leads to the overproduction of very low-density lipoproteins (VLDLs) rich in triglycerides. This results in the accumulation of triglycerides within hepatocytes, contributing to hepatic steatosis [37, 38]. Furthermore, IR drives the secretion of larger, triglyceride-enriched VLDL particles and significantly reduces HDL-C concentrations, exacerbating lipid imbalances [39]. Recent studies have also highlighted a strong association between IR and leptin secretion [40]. Persistent hyperleptinemia has been linked to fatty liver, hepatic fibrosis, and hepatocellular carcinoma in several observational clinical studies, suggesting it may be a reliable marker for the onset or progression of NAFLD [41, 42]. Currently, leptin is believed to contribute to NAFLD development by centrally inhibiting food intake while enhancing sympathetic nervous activity, metabolic rate, and gluconeogenesis [43, 44].
In addition to lipid abnormalities, oxidative stress and inflammation play pivotal roles in the progression of NAFLD to more severe liver diseases. Oxidized low-density lipoprotein (oxLDL) is a key mediator of these pathogenic processes. A recent study by Ampuero et al. revealed that the oxLDL antibodies/HDL-C ratio was significantly higher in lean NAFLD patients [45]. The accumulation of oxLDL in the liver triggers a cascade of inflammatory responses, including the activation of hepatic macrophages (Kupffer cells) and the secretion of pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6). These cytokines exacerbate insulin resistance, creating a feedback loop that leads to further lipid accumulation and hepatocellular damage [46, 47]. The oxidative stress associated with NAFLD has been shown to contribute to mitochondrial dysfunction and lipid peroxidation, with the overproduction of reactive oxygen species (ROS) further amplifying cellular injury and promoting the progression of liver disease [48]. Emerging evidence suggests that excess intracellular cholesterol activates liver X receptors (LXRs), leading to increased VLDL secretion and the formation of small, dense LDL particles. These particles are more prone to oxidation and have reduced affinity for LDL receptors, thereby enhancing their atherogenic potential and contributing to the pro-inflammatory environment within the liver [49]. Additionally, Mocciaro has identified low levels of HDL-C as a risk factor for NAFLD. Low HDL-C levels reduce the body's ability to transport cholesterol back to the liver, making LDL cholesterol more prone to oxidation [50], a finding that aligns with the study conducted by Karami [51].
Existing research has indicated that NHHR is a better predictor than non-HDL-C for cardiovascular disease (CVD) in individuals with type 2 diabetes [52], and it is also a superior predictor for IR and MS compared to the apoB/apoA1 ratio [53]. Two studies have shown that NHHR outperforms non-HDL-C as an independent predictor of new-onset NAFLD in both Chinese adults and adolescents [54, 55]. Therefore, it is essential to investigate the association between NHHR and NAFLD and hepatic fibrosis in the general American adult population. Our study aligns with previous research and contributes to the understanding of the causal relationship between dyslipidemia and NAFLD.
Strengths and limitations
The strengths of this study include its large sample size and the relatively reliable assessments of hepatic steatosis and fibrosis. Notably, this is the first study to identify a positive association between NHHR and both NAFLD and liver fibrosis in the general American population. Given its simplicity and ease of determination, NHHR may prove valuable in identifying NAFLD in adults. However, several limitations must be acknowledged. First, the cross-sectional design limits the ability to draw definitive causal inferences between NHHR and NAFLD or liver fibrosis. Future longitudinal studies are necessary to better explore potential causal relationships. Second, as this study was conducted exclusively in American adults, the generalizability of the findings to other populations or ethnic groups is uncertain due to genetic and environmental differences. Third, while liver biopsy is the gold standard for grading hepatic steatosis and fibrosis, NAFLD in this study was diagnosed using ultrasonography rather than histopathology, which may introduce diagnostic inaccuracies. Finally, due to limitations of the NHANES database, some confounding factors that could influence the results were not accounted for. Therefore, further prospective clinical and basic research is needed to validate these findings and address the aforementioned limitations.